The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Conductive inks have been used for several decades to form conductive traces on a variety of flat, two dimensional (2-D) substrates or articles, such as circuit boards and flexible plastic films. One of the more common methods of depositing these inks onto these articles is screen printing.
In order to print on more complex substrate geometries that have a three-dimensional (3-D) shape, however, other print methods, such as pad printing, must be utilized. In pad printing, the ink is applied as a pattern to the surface of a conformable pad, which then makes contact with the 3-D surface of the substrate in order to transfer the ink. However, even pad printing cannot be utilized in some applications where the substrate geometry is more complex.
In this case, a print head that ejects drops of ink from a distance above the substrate's surface is utilized. In order to print a substrate that has a very complex 3-D geometry, a large distance between the print head and the substrate is desirable. Commercially available print heads generally contain a nozzle or orifice through which ink is ejected in discrete drops, one at a time, as well as a valve that regulates the flow of the ink. Pressure is applied to help force the ink through the nozzle. The closing action associated with the valve also produces a significant force, which assists in ejecting the drops from the nozzle. When the valve is opened and closed in rapid succession, a stream of drops can be produced.
The method of printing that uses a print head to eject drops of ink onto a substrate should be distinguished from the method in which ink flows from a channel or other orifice that is in close proximity to the substrate's surface as a continuous filament of ink. In addition, this method of printing should also be distinguished from inkjet printing, which requires inks with very specific rheological properties, for example, less than 0.02 pascal second (Pa-s) in viscosity.